In utility and industrial boilers burning coal, fly ash is produced. Electrostatic precipitators, wet scrubbers, bag houses or other recovery equipment are used to remove the fly ash particulates from the combustion gases. In general, the fly ash is composed of fine grained particles having a silica-alumina base with certain of trace metals concentrated on the surfaces of the particles. Some of these trace metals are valuable, including molybdenum, tungsten, nickel, gallium, and germanium. Others, such as arsenic, lead and antimony, are toxic.
Numerous attempts have been made in the past to recover certain of the trace metals present in fly ash, particularly gallium and germanium. One such method is the sublimation of these trace metals. The process involved the following steps: (1) the fly ash is pelletized and then dried and hardened so that the pellets can be stacked on top of each other within the furnace; (2) the pellets are then heated in a furnace in an oxidizing atmosphere; (3) the pellets are then subjected to a reducing atmosphere, causing the gallium and/or germanium to sublime and be carried away by the reducing gas--thereby separating the sublimate from the pellet residue; (4) the sublimate is then recovered from the reducing gas, the reducing gas being reused in the reducing atmosphere; and (5) the pellet residue is cooled and removed from the furnace.
It is therefore an object of this invention to provide an improved furnace arrangement for the recovery of trace metals from fly ash.
Another object of this invention is to provide a furnace arrangement capable of continuous operation in the recovery of the trace metals from fly ash.
Yet another object of the invention is to reduce the number of separate steps necessary to recover trace metals from fly ash, thereby decreasing the cost and improving the efficiency of the process.